Lithium ion secondary batteries are widely used for portable electronic instruments such as cell phones and notebook-size personal computers. As a cathode active material for a lithium ion secondary battery, a cathode active material comprising a composite oxide containing Li and a transition metal element (such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2 or LiMn2O4) has been known. For example, a lithium ion secondary battery using LiCoO2 as a cathode active material, and using as an anode a lithium alloy, graphite, carbon fibers or the like, which achieves a high voltage of about 4V, is widely used as a battery having a high energy density.
For a lithium ion secondary battery for portable electronic instruments or vehicles, it is desired to reduce the size and weight, and it is desired to further improve the discharge capacity per unit mass (hereinafter referred to simply as “discharge capacity”) and such characteristics that the discharge capacity and the average discharge voltage will not substantially decrease after repeating the charge and discharge cycle (hereinafter sometimes referred to as “cycle characteristics”).
As a cathode active material having a high discharge capacity, a cathode active material comprising a composite oxide having a high Li ratio relative to the transition metal element (hereinafter sometimes referred to as “Li rich cathode active material”) such as the following cathode active material (i) has attracted attention.
(i) A cathode active material which contains a solid solution of a lithium transition metal composite oxide having an α-NaFeO2 crystal structure, wherein the compositional ratio of Li and the transition metal elements contained in the solid solution satisfies a compositional formula of Li1+1/3xCo1−x−yNiy/2Mn2x/3+y/2 (wherein x+y≤1, 0≤y and 1/3<x≤2/3) (Patent Document 1).
However, from the cathode active material (i), Mn is likely to elute into an electrolytic solution by contact with a decomposed product formed from the electrolytic solution by charging at a high voltage. Thus, the crystal structure of the cathode active material (i) tends to be unstable, and no sufficient cycle characteristics are likely to be obtained.
Accordingly, in order to improve the cycle characteristics, the following cathode active materials (ii) and (iii) have been proposed.
(ii) A cathode active material comprising a cathode active material and a layer containing P formed on the surface layer of the cathode active material by bringing the cathode active material into contact with a lithium dihydrogen phosphate solution or a diammonium hydrogen phosphate solution (Patent Document 2).
(iii) A cathode active material having a phosphorus compound incorporated in the vicinity of the surface of a lithium composite oxide, obtained by mixing an oxide or hydroxide containing a transition metal element, a lithium salt and a phosphorus compound containing at least one of PO3 and PO4 and firing the mixture (Patent Document 3).
However, the cathode active materials (ii) and (iii) hardly achieve sufficient cycle characteristics when formed into Li rich cathode active materials.